For planar printing substrates, such as paper products, there is a need for hiding power and visual appearance uniformity. Optical opacifiers, such as titanium dioxide (TiO2), are used to provide these attributes. A highly uniform front surface appearance is desired in these products, even when there is printing on the reverse surface of the product. The degree to which images printed on the reverse surface can be seen when the front surface is illuminated and viewed is known as “showthrough”. It is well known that the thickness and the inherent opacity of the product, as well as the penetration depth of the ink printed on the reverse surface, affect the degree of showthrough.
Showthrough may be evaluated by trained human operators who make subjective ratings of the surface appearance based on visual observations of the front surface under controlled lighting conditions. Opacity measuring instruments, known as opacimeters, have been traditionally utilized to quantify the degree of showthrough within a single small region on the surface. A standard measurement protocol, using such an instrument, is set forth in Technical Association of the Pulp and Paper Industry (TAPPI) standard T-425 om 01, entitled “Opacity of Paper (15/d Geometry, Illuminant A/2 Degrees, 89% Reflectance Backing and Paper Backing)”. Measurements made with such instruments, however, measure a very small portion of the overall area of a sample sheet and are thus ill-suited to provide a representative measure of showthrough, particularly when a patterned, image, or indicia, is printed on the reverse side of the product. An automated imaging system and method, such as that of the present invention, should provide a more representative measure of showthrough.
A method for quantifying visual appearance uniformity of planar surfaces of opaque objects is described in U.S. Pat. No. 6,438,256 (assigned to the assignee of the present invention). This method, however, does not address the need for assessing the visual showthrough of printed images on the reverse side of a sheet of paper.